1. Field of the Invention
This invention relates to microspheroids composed of a plurality of colloidal particles and to a process for their manufacture. Such microspheroids are useful as column packings for chromatography, particularly liquid chromatography, as catalysts or catalyst bases, and as pigments.
2. Discussion of the Prior Art
U.S. Pat. No. 3,505,785 discloses superficially porous microspheroids, having an average diameter in the range of 5 to 500 microns, which are composed of an impervious core coated with a multiplicity of monolayers of colloidal inorganic particles having an average size in the range 0.005 to 1.0 micron. These superficially porous particles have a limited number of pores which are not distributed uniformly throughout the microspheroid.
U.S. Pat. No. 3,855,172 discloses microspheroids which are porous throughout and have an average diameter in the range of 0.5 to 20 microns. They are composed of colloidal inorganic refractory particles having an average diameter in the range of 0.005 to 1.0 micron. These microspheroids, which are characterized by their extremely uniform size, are formed by a coacervation process in which the colloidal particles are first encapsulated in an organic polymer matrix and then formed into porous microspheroids by burning off the organic material. For microspheroids in which the pores are uniformly distributed throughout the microspheroid, the average pore diameter is roughly equal to one-half the average diameter of the colloidal particles which make up the microspheroid. The pore diameter of such microspheroids can then be controlled by properly choosing the diameter of the colloidal particles used to form the microspheroid.
With the organic material present, microspheroids composed of large colloidal particles in the size range of 0.1 to 1.0 micron can be formed by the process disclosed in U.S. Pat. No. 3,855,172. When the organic material is burned off, however, these particles are extremely fragile and have limited use, unless the burning off takes place at a high enough temperature to sinter the colloidal particles together. When this occurs, the surface area of the microspheroid decreases markedly below that of the particles from which it is formed and the average pore diameter decreases, ultimately yielding a non-porous particle. There is, therefore, limited control over pore size and pore volume when larger collodidal particles are used to form the microspheroids.
U.S. Pat. No. 3,920,578, which issued on Nov. 18, 1975, to P. C. Yates et al., discloses an aqueous composition which can be used as one of the starting materials to form the powder of the present invention. As disclosed in Example 2 of that patent, a sol of large colloidal silica particles and soluble silicate is prepared. This sol will gel to a structure with 4.8% by weight of small particles and 95.2% by weight of large particles. However, microspheroids are not formed and the structure is not converted to the macroporous structure of the present invention.